Process for producing interlaminar insulation for electrical apparatus



United States Patent 3,129,124 PROCESS FOR PRQDUCING INTERLAMlNAR INSULATIUN FOR ELECTRICAL APPARATU Duane L. Barney, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Dec. 30, 1959,. Ser. No. 862,765 1 Claim. (Cl. 1486.35)

The invention described herein relates to insulation and more particularly to a process for forming a thin layer of insulation on magnetic and non-magnetic metallic bodies. A specific application of the invention resides in insulating the surfaces of silicon steel laminations used in electrical apparatus, such as motors, generators, transformers, and the like.

Both organic and inorganic materials are used for providing a thin layer of insulation on metallic objects, such as silicon steel laminations used in electrical apparatus, but the methods practiced in acquiring establishment of the dielectric film are expensive and the resulting product, if organic, cannot be used in relatively high temperature environments.

Various types of resins, for example, phenolic varnishes, currently are employed as insulation agents. These and other well known organic materials usually are rolled, dipped or sprayed on the metal surfaces thus requiring expensive equipment to carry out the process. Considerable time is involved in handling the product, both during and after practicing the process, so that labor costs generally are very high. Another important disadvantage connected with use of organic materials resides in their decomposition point characteristics, usually in the neighborhood of about 300 C. so that if a dynamoelectric machine for example, is operated in a temperature range where conversion to a viscous or even a charred state occurs, the dielectric strength previously offered by the insulating film is substantially diminished. Moreover, if the laminated product is a magnetic core maintained under pressure, loss of the insulating medium will result in looseness in the core stack with correspond ing deleterious effects on the machine.

Inorganic insulating materials likewise are sprayed or otherwise deposited on the metallic surfaces for dielectric purposes and these broadly include phosphates and silicates in liquid form. Many of the processes which utilize inorganic materials of the type described above rely on a chemical conversion process wherein the surface of the base metal is converted to another compound. For example, a well known process includes applying a phosphoric acid solution to a silicon steel lamination for establishing an insulating coating of iron phosphate which displays good temperature stability and which usually melts at temperatures approximately 500 C. The primary objection to processes involving inorganic compounds is the expense involved in practicing the various steps required for insulating the metal. Not only is special equipment required but considerable time which represents labor costs, is needed to carry out the process.

Still other processes utilize a gas, such as oxygen, for establishing an oxide coating on metal objects, which in some cases, provides acceptable insulating characteristics. The principal drawback here is that a thick coating with sufiicient stability to serve as a durable insulation medium is not attainable. Ammonia has been suggested for nitriding or hardening parts and fluorine may prove useful, but in the latter case, the toxicity aspects preclude its use as a practical matter.

Although the above practices produce insulation coatings having acceptable dielectric and mechanical characteristics, they contain cost disadvantages and do not permit use of insulated products under a wide range of applications.

3,129,124 Patented Apr. 14, 1964 ice It therefore is an object of my invention to provide an insulated metallic object having an insulation of high dielectric strength on its surface which displays good temperature stability at temperatures in excess of 500 C.

Still another object of my invention is to provide a process for furnishing an insulating coating on a metal surface by using a gas harmless at ambient temperatures, but sulficiently reactive at high temperatures to produce an insulation of high dielectric and mechanical strength.

In carrying out my invention in one form, I expose a single or plurality of metallic objects to a gas containing fluorine atoms at an elevated temperature for a period of time sufficient to establish a thin film of insulation on the metal surface. The insulating film is tough and hard, displays good temperature stability and contains excellent dielectric characteristics. It will readily occur to those skilled in the art that the invention has application to many different and diverse metals and that the temperature range and times used in obtaining conversion of the metal surface to an insulating coating may vary according to the various types of metals used.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the coneluding portion of this specification. My invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following disclosure.

During the investigations leading up to development of this invention, the specific problem presented was one of finding an inexpensive method for obtaining a higher quality insulating film on the surface of silicon steel laminations. However, the areas of exploration of necessity encompassed many different types of metals, such as, iron, steel, aluminum, copper, nickel, and the like, because of their wide usage in induction apparatus and particularly in dynamoelectric machines. To facilitate an understanding of this invention, the following disclosure will be directed to silicon steel laminations although it will be both apparent and understood that the invention has application to metals other than those specifically described.

The known liquid or solid products heretofore used as insulating agents do not contain all the attributes considered desirable for electrical insulation purposes. The main disadvantage is that they cannot withstand high temperatures. Also problems connected with storage and mixing such products when they must be placed in a liquid form involves time and effort which increases the labor costs involved in insulating silicon steel laminations. Analysis of the various types of gases used for depositing a thin film of insulation on metallic objects showed that although insulating films were provided by such gases, they were not sufficiently thick to furnish the desired degree of mechanical and dielectric strength.

Nevertheless, great benefits can be derived from using a gas thereby making it highly desirable for insulation purposes. The gas can be stored conveniently in flasks or other objects without the necessity for taking excessive precautionary measures for protection of personnel. The means used for connecting the supply of gas with a receptacle in which the metallic objects would be located usually involves nothing more than ordinary piping connections. Moreover, it can be confined in closed receptacles during the process of forming a thin film of insulation on the metallic objects.

Preferably, the desired attributes for such a gas would be commercial availability and low cost, and one which does not require apparatus of special design to permit passing it in contact with the metal parts to be insulated. it should be harmless and 'be capable of providing insulation having stability at high temperatures including high mechanical and dielectric qualities to permit its use in electrical environments. In those instances where the metallic part to be coated also must be subjected to an annealing cycle, the gas preferably should react with the metal at annealing temperatures, i.e., in the neighborhood of 700 C. In this situation it will be seen that the two separate steps previously performed of annealing, then insulating, could be performed by one step.

The only disadvantage of real significance lies in the toxicity aspects of some gases which may be used for insulation purposes. Those gases toxic at room temperature are considered hazardous and their use generally should be avoided. At a minimum, the gas should not be toxic at room temperatures, or at a temperature other than that encountered during the step of forming the insulation on the metallic surface.

Although the above requirements appear to be stringent, a gas having the desired characteristics has been found which will effectively provide a high quality layer of insulation on metallic surfaces. Such a gas is sulfur hexafiuoride. At ordinary temperatures, sulfur hexafluoride is not toxic and is extremely stable chemically. Available technical literature states that it behaves more like nitrogen than like a halide of sulfur. However, at elevated temperatures of about 400-500" C., it does participate in reactions which have as their by-products, sulfur and fluorine containing gases which are toxic in a raw state. This drawback of toxicity at high temperatures is not as severe as one might expect however, since the toxic gases can be trapped easily and effectively with activated alumina. Conventional apparatus used for this purpose is connected directly to the oven so that the harmful gases are led directly into the gas entrapping device.

Inhaling SP should produce no harmful effect as long as the 20 percent oxygen content of the air is preserved. However, its decomposition products should be avoided. SP breaks down into SP SP S /F and possibly S 1 The first two, SP and SP each hydrolyze with atmospheric moisture to give HF and S Although HP is poisonous when breathed, it is easily detected and so can be easily avoided. The highly toxic SgFm decomposes at temperatures below those at which SP decomposes. Any decomposition products of SP can be absorbed as they are formed by potassium hydroxide or the activated alumrna.

Up to 200 C., SR; is extremely stable. In quartz containers, it is unaffected by temperatures as high as 500 C. In any known container, it is stable at temperatures above which oil begins to oxidize and decompose. But in the presence of some metals, it may slowly break down at temperatures in excess of 200 C.

Upon exposure to an open arc, the gas will slowly decompose. A test, using a two-foot length of 2.2 x 1.0 cm. waveguide, indicated that about percent of the gas contained in the guide had decomposed after five hours exposure to continuous arcing. Decomposition products may attack certain metals. Table I lists some common metals and their order of resistance to attack.

Table I .Resistance of Metals to Decomposition Products Silver-most resistant Aluminum Stainless steel Copper Brass Steel Silicon steel-least resistant An indication of the desirable characteristics imparted to the surface of metallic objects by passing sulfur hexafiuoride in contact therewith is illustrated by the follow- Silicon steel laminations of the type conventionally used in dynamoelectric machines were placed in a steel receptacle and a stainless steel tube interconnected a supply of sulfur hexafluoride gas with the receptacle. The laminations then were heated to a temperature between 675 C. and 760 C. and the gas introduced into the heated receptacle. It was found that after subjecting the laminations to the gas at this temperature for 10 minutes, followed by cooling, the surface of the laminations were converted into a green insulating coating which was merged directly in the base metal. By X-ray diffraction, it was found that the coating was ferric fluoride. The thin film of insulating material thus formed was approximately 0.5 mil in thickness. When subjected to a 6-volt potential it was found that upon measurement of the dielectric strength, the insulation resistance was over 20 megohns. By varying the time at which the silicon steel is exposed to the gas, the insulation accordingly can be varied in proportional amounts.

It is not definitely known what the decomposition products are in the sulfur hexafluoride gas which react with the metal surface at these high temperatures to produce the hard coating of ferric fluoride having the desirable characteristics mentioned above. It is believed however that the following reactions take place.

The sulfur hexafiuoride and iron combine when subjected to these high temperatures to produce at least the following products: ferric fluoride, sulfur fluoride, sulfur tetrafluoride and sulfur decafluoride. The sulfur fluoride and sulfur tetrafluoride react with water to hydrolyze and produce hydrogen fluoride and sulfur dioxide. It is suspected that many different reactions take place during the process but the final reaction product appearing in the form of insulation material definitely can be measured and exhibits the desirable characteristics mentioned above. During the reaction, the stainless steel receptacle was coated also with an insulation film of approximately 0.5 mil thickness, thus indicating that the gas also is capable of reacting with stainless steel for providing a thin film of insulation having the same characteristics as that previously mentioned.

Aluminum discs can be placed in a closed reaction vessel and heated to a temperature between 675 and 750 C. and subjected to the influence of sulfur hexafluoride to produce a coating of insulation on the surface of the aluminum. In order to vary the thickness of the insulating film the reaction time is varied from the above ten minute heating cycle to obtain either a greater or lesser thickness of coating. For example, a five minute reaction time will produce a very acceptable insulating film.

Copper discs can be placed in a closed reaction vessel and heated to a temperature between 675 and 750 C. and subjected to the influence of sulfur hcxafluoride to produce a 0.5 mil coating of insulation on the surface of the copper. Variations in insulation thickness is accomplished by changing the reaction times.

Specimens of nickel can be placed in an oven and heated in a temperature range of 675-750 C. in the presence of sulfur hexafluoride for approximately ten minutes to provide a dielectric coating of approximately 0.5 mil thickness on its surface. The thickness of the nickel fluoride coating can be varied by changing the reaction time, a thinner insulating layer being obtained by decreasing the above time period.

In view of the above it will be evident that many variations are possible in light of the above teachings. Although specific examples of metals have been used for illustrating the invention, it will be evident that other metals capable of reacting with sulfur hexafiuoride will provide a similar coating of insulation. Such metals constitute those elements which are electro-positive with respect to fluorine.

The reaction between elements comprising the gas and the metal surface takes place rapidly in the temperature range of 675-750 C. but the reaction is not confined to this specific temperature range. It is believed to commence at temperatures starting about 500 C. and extends up to a temperature point where the physical properties are detrimentally affected. Therefore, the invention may be practiced within the broader range of temperatures and especially in those situations where the metal object must be heated to a temperature falling within the broader range of temperature.

Although sulfur hexafluoride has been disclosed specifically as constituting the primary reaction agent, other fluoride gases will produce similar coatings of insulation. Such gases may include sulfur difluoride, sulfur decafluoride, hydrogen fluoride and sulfur dioxide obtained by reacting sulfur difiuoride and sulfur tetrafiuoride with water in air. Obviously other suitable fluoride gases may be used.

It therefore is to be understood that within the scope of the appended claim, the invention may be practiced otherwise than as specifically described.

What I claim as new and desire to secure by Letters Patent of the United States is:

A process for establishing a thin film of insulation on a silicon steel lamination comprising the steps of placing said lamination in a closed receptacle, introducing sulfur hexafluoride gas in the receptacle and circulating it in contact with the silicon steel lamination, heating the lamination in said receptacle to a temperature between 675 and 750 C. for a period of time sufiicient to cause a chemical reaction between the gas and the metal surface and thereby form a thin layer of insulation on the metal surface, said insulation comprising a fluoride of the gas and the base metal.

References Cited in the file of this patent OTHER REFERENCES Simons: Fluorine Chemistry, vol. 1, 1950, Academic Press Inc., Pub., New York, N.Y.; pages 31, 40, 64, 68 and 90-92 relied on. 

